Biospace wrote: ↑12 Aug 2024, 11:33am
Cugel wrote: ↑10 Aug 2024, 1:34pm
I'd be interested to know what sort of braking drag could be produced by bike wheel hub dynamo style brakes that would primarily be useful for braking down those very long and bendy descents, where one doesn't want to be heating up the rims or the discs. Would the maths show that a significant braking effect down, say, 3 miles of such a descent (of, say, 300 metres drop) on a laden e-tourer would also produce an amount of power significant enough to add a useful top-up to the battery?
In our e-car the regenerative braking is used for 95% of the braking, which can be significant in an almost daily drop from 400 metres above sea level to 100 metres, down narrow and twisty back roads from Fforest Brechfa dog walks into the Teifi valley where we live. The regen braking output can take the battery level from, for instance, 30% back up to 50% in the 13.8Kwh battery - somewhere around 3Kwh generated*.
So ..... regen braking can be significant in energy production with a heavy car. Does it scale to a heavy bike, though (weighing much less than a car but also needing much less energy capacity in the battery).
* The best approach to get the most charge back into the battery from e-car regen braking is to go fairly slowly. Above a certain road speed (about 40 kph in ours) down a steep hill, the maximum recharging rate of the battery from the regen braking is exceeded. If the road speed is kept at, say, 33 kph, all of the energy produced by the regen braking seems to get put back into the battery and the braking is very noticeable. Above 40 kph the perceived degree of braking drops and one has to apply the disc brakes too, despite the graph showing the regen braking charge rate remaining high. Presumably the excess charge from the motors-as-generators is then wasted instead, as heat.
"Hub dynamo style brakes" - I'm presuming you're thinking of a hub motor, in which case with the right sort of motor I'd expect ~40% of max output power to be available for braking, with the correct software and battery capabilities. With regen brakes, once the battery is full or nearly full, there's little or no electrical braking unless there's a dynamic heat dump also.
For me, unless I regularly rode a long descent as described in my previous post, all this adds complexity which wouldn't deliver sufficient return for it to be worthwhile.
The threshold of 40kph in your EV is interesting, since the faster you go the less braking is needed to maintain the same speed (air drag) - it's most likely a safety thing. Since you observe these things carefully and note a return from 30% to 50% capacity over the 300m descent, what is the capacity loss in battery on the way up, if you ascend using battery alone?
Bike regen hub brake - yesterday's ride along the logging roads in Fforest Brechfa provided a perfek example of where a back brake regen brake would have been useful. On most of the often long descents, I needed to use the back brake a lot of the time to stop the bike running up to unmanageable speeds on the rough and often blind-bended tracks. (The front brake was only used on the most grippy sections). A rear hub regen brake that could have been set for a constant drag would have kept the gravity speed increases in check with a significant reduction in the likelihood of a skid .... and a bit of battery recharge.
It might be the case that such a regen brake would put back nearly all of the battery charge that I use to help get up the various very steep sections. I only use the e-motor sporadically for the hardest ascents .... around 40 watt-hours of the battery's 250 watt-hour full charge over yesterday's near 50 km + 1000 metres of climbing (and descent) ride.
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The e-car is a hybrid, although it runs on electricity 95% of the time (according to its computer thingy) as we rarely make journeys of longer than its 30 mile e-range before it gets recharged from the solar panels. The range is nominally 28 or 30 miles, depending on which Mitsubishi spec you read. In practice it varies a great deal with how much climbing is done, the average speed of a journey, the wind direction and several other factors.
It has a display that supposedly tells you the miles left in the battery as well as showing a bar graph of the charge left. It's rightly known as "the guessometer" though.
However, over hundreds of short journeys (6 - 12 miles) up and down to & from Fforest Brechfa the guessometer typically shows 50 - 70% battery used to go up (200 - 300 metres) and 15 - 20% put back on the way down. I do drive very carefully, generally always below 40 mph and usually more like 25mph or less on the narrow & twisty backroads. (And now, 20 mph through the villages).
I rarely use the disc brakes (typically only for junctions, sudden events such as a cat leaping out of a hedge bottom or to stop altogether) as the 5 selectable different levels of "drag" braking from the regen facility can control road speeds all by themselves .... as long as those speeds are relatively low.
“Practical men who believe themselves to be quite exempt from any intellectual influence are usually the slaves of some defunct economist”.
John Maynard Keynes